Coherent light sources (light sources for producing coherent light) such as laser light sources are used in image display devices, measuring instruments, exposure lithographic devices used for producing a circuit on a semiconductor substrate and the like. As compared with lamp light sources used in projectors and the like, laser light sources using lasers can more easily increase light utilization efficiency since having longer lives, higher emission efficiencies and stronger directionalities. In the case of being used in an image display device, a laser light source can display a sharp image with a large color reproduction region since being monochromatic.
On the other hand, speckle noise is produced in image display devices, measuring instruments, exposure devices and lighting devices using laser light sources as light sources. For example, when an observer views a virtual image formed by an image display device, rays of light from the respective regions of a screen overlap on the retinas of the observer to form an image. At this time, since the rays of light by way of different regions overlap on this screen in a complicated positional relationship and laser light has high coherency, the rays of light interfere with each other. Since an interference pattern is formed by such interference, it reduces the quality of a displayed image as an unintended light intensity distribution (speckle noise). Similarly, in the case of using a laser light source as a light source for a measuring instrument or an exposure device, light intensity becomes uneven on a target surface due to the coherency of laser light.
The above speckle noise is a common problem in the case of using coherent laser light as a light source for an image display device, a measuring instrument, an exposure device or the like, and various attempts have been made thus far to reduce the speckle noise.
For example, constructions using a rotating diffuser have been proposed as disclosed in patent literatures 1, 2. In these constructions, a diffuser is arranged on an optical path of laser light emitted from a light source and rotated at a high speed, and the laser light passes through the diffuser. An interference pattern produced by the laser light that is coherent light is disrupted by rotating this diffuser at a high speed, and this interference pattern is averaged by being moved around at a high speed on the screen, whereby the speckle noise is reduced.
In other words, in these constructions, the interference pattern does not actually disappear, but it looks as if the speckle noise had disappeared by superimposing a plurality of different (having no correlation with each other) interference patterns.
However, since these constructions require a large mechanical rotating mechanism, a vibrating mechanism whose amplitude exceeds 100 μm, and the like in these constructions, noise is produced. Further, since large power is consumed for driving, power consumption of the entire device increases.